Chainplates life expectancy

[rhythmsurfer]

Just wondering on what the concensus is on replacing chainplates. This chainplate came off of my 1981 H36 today. I am rebedding the chainplates due to the water sitting in the fiberglass folds where the base of the plates bolt. While I have them out I took these pictures. I was thinking of just taking a brillo pad to the plates and then maybe coating them with a light coat of oil. Let me know what you think

 

[woodster]

While I have them out I took these pictures. I was thinking of just taking a brillo pad to the plates and then maybe coating them with a light coat of oil. Let me know what you think

personally ....i wouldnt use a brillo pad unless it is made of stainless steel....might be better if you use a scotch brite/burgandy pad...or even use 600 grit sand paper....i am assuming they are stanless steel.....as for the light coat of oil i would use boeing shield or some thing like it....hope this helps...

regards
woody

 

[kloudie1]

I’d lightly brush it with a stainless brush in your drill. Then, inspect carefully for cracks. I would use a magnifying glass to inspect.. The place that looks the worst to me is on the left side of the picture where the rust stains are. I would take that to a welding shop and ask them to Dye Penetrant (PT) inspect that area for cracks. If no cracks, and the stuff is pretty much cosmetic, there should be no problem in continuing to use these.. A case of crevice corrosion or a crack would be the only reason for condemning the straps. Crevice corrosion would look like a gray pitted and eroded area, most likely at the deck penetration. A small dot of it here and there is OK, but not a line that would cross the strap completely. If you are not comfortable with doing the inspection, do take it to a welding shop and have those guys do it. There are a couple of shops on Bayou Chico that could do the inspection and fab a new one if necessary.

 

[Ross]

First put a magnet on them. They should be non-magnetic. Then I would get agressive with some flexible abrasive cloth and clean up the stains. Then you can check for subsurface damage with a dye penetrant. Rust stains can come from many sources besides the piece you are looking at.

 

[Prime Time]

I pulled a chain plate and sent it to a rigger (Brian Toss) for inspection with microscopic techniques when I was re-rigging the boat. It was not very expensive. In an article that I read, chain plate failure is one of the leading causes of rig failure. Given the rust present on yours, I'd be very worried.

Having the stainless highly polished aids in preventing corrosion.

 

[PeteK]

As for coating the sstl before reinstall, remember that "stainless" is only that
way when there is oxygen available to form the chromium oxides. If the bedding
or coating blocks oxygen then crevice corrosion will occur.

Pete

 

[Ross]

As for coating the sstl before reinstall, remember that "stainless" is only that
way when there is oxygen available to form the chromium oxides. If the bedding
or coating blocks oxygen then crevice corrosion will occur.

Pete

This is true if an electrolite is present. If the bedding prevents the intrusion of salt water then the won't be a problem.

 

[RichH]

Id be VERY concerned about that 'band' of rust. Usually indicates both crevice corrosion and fatigue fracturing - simultaneously.
To be sure and BEFORE you 'clean' the rust off --- Find an automotive machine shop who does 'magnaflux' analysis, either magnetic or dye penetrant, and have the plates 'assayed'. If 'any' cracking is discovered by such analysis - replace them.

 

[rhythmsurfer]

Thank you so much for the replies. I will follow through. Hopefully they will pass and if not....hopefully they won't be too expensive to replace.

 

[RichH]

The 'technical' answer for chainplate life expectancy is: *in the absence of crevice corrosion*, one can expect 1 million load cycles at approximately 30000 psi stress (1/3 of the ultimate tensile values of 300 series stainless steel) before the material becomes 'embrittled' (fatigued).
In most cases this would equate to approximately one 'circumnavigation' before the plates 'simply fall apart' due to 'fatigue'.

The widespread/common error in design with stainless steel for repetetive stress loaded applications is to design for 'ductile' failure (90,000 psi) rather than 'fatigue' failure (30,000 psi / ~1M million load cycles) and not to include ample 'safety factors' which further 'de-rate' the design load bearing capability of the material to insure that material doesnt 'fatigue' and thus extends the service life.

Fatigue in stainless steel begins at the 'first' few load cycles beyond that 1/3 ultimate tensile value and is additive; however, once 'microscopic cracking' begins corrosion can quickly accelerate into those 'cracks'; thus, you have two simultaneous failure modes going on at the very same time which can radically/exponentially shorten the 'life in service'. "Rusting" on such stainless components is a signal that dangerous 'catastrophic failure' is probably beginning to occur.

 

[Ross]

My chainplates have a cross sectional area of .375 square inches. To load them to the 30,000 PSI level would take just over 11,000 pounds. My shrouds have a breaking strength of about 9,000 pounds. Therefore I can never fatigue my chainplates to failure.

 

[Prime Time]

Interesting Article

http://www.dixielandmarine.com/yachts/DLrigprob.html

This may shed some light on the issue.

 

[RichH]

Ross -- good point but not totally correct to the nth degree.

There will be 'impacts' and unforseen 'overloads', plus you do have 'drill holes', sharp edges and probably unpolished plates which also lower the fatigue and service life ... so therefore not 'never' but maybe the 6th owner after you will need to replace them. Of course if you see any 'blooming rust' on the plates suggesting crevice corrosion which greatly and progressively reduces the effective 'cross section' -- then all bets are off.
;-)

 

[Ross]

I bought 316L SS from my brothers steel supplier when I made the plates and champhered all of the holes. I figure that they will outlast several sets of standing rigging.

 

[rhythmsurfer]

Prime Time thank you for the link. Ross and RichH thank you for your concerns. I work in an ER and as a flight paramedic so I usually see the end result of carelessness and stupidity. I don't want to lose a mast and have it hit somebody in the head or lose the mast in heavy seas. So...I am trying to be careful in my retrofit.

 

[shemandr]

Isn't most of the cost replacing chain plates the labor. You've already done that. Why not put new back in.
As PeteK said, stainless needs Oxygen contact to remain rust free - so the oil coat would seem to be counterproductive.

 

[Ross]

A coating of oil will not be durable. If you plan to use any coating just use carbon steel and keep it painted.

 

[Steve Dion]

Do we have any idea on the cost of replacing these uint?

 

[Scott T-Bird]

Personally, I wouldn't even think of testing them ... I'd just replace them. Besides, the economy could use your money.

 

[Ross]

I agree with Scott if you have any doubts then change the chainplates. You don't need to worry about the quality of your boat during a storm. Will they hold? Will that anchor cleat be strong enough? Will that repair I made to the bilge pump plumbing fail?
Nothing too strong ever broke.